1. Field of the Invention
The present invention relates to an infrared sensor that is constituted of a substrate having a membrane and a thermo couple.
2. Description of the Related Art
JP-A-7-181082 discloses an infrared sensor that is constituted of a base plate having a membrane formed therein and a thermocouple unit located on the base plate. As shown in FIG. 6, the base plate 1 is constituted of a silicon substrate 1a, an insulation coating 2 formed on the silicon substrate 1a and an infrared radiation absorbing member 3. The central portion of the silicon substrate 1a is thinned by etching to form the membrane Ma. Such an infrared sensor detects an incident infrared radiation at the membrane Ma by the thermocouple unit T, which is so called a thermopile. The thermopile is constituted of a plurality of couples of thermocouple elements (e.g., Tm and Tn shown in FIG. 1) that are connected in series. Each couple of the thermocouple elements is constituted of a hot side contact Th and a cool side contact Tc and arranged to detect an electromotive force generated between the contacts Th and Tc. The hot side contact Th is disposed on the membrane Ma and the cool side contact is disposed on a portion of the silicon substrate 1a outside the membrane Ma, where the heat capacity is much larger than the heat capacity of the membrane. The infrared radiation absorbing member 3, which is made of a carbon-mixed resin, is disposed to cover the hot side contact Th. One thermocouple element is made of an aluminum film, and the other thermocouple element is made of a polycrystalline silicon film.
When the infrared radiation absorbing member 3 absorbs an infrared radiation, the temperature of the hot side contact Th, which is disposed on the membrane Ma, becomes much higher than the cool side contact Tc, because the membrane has much smaller heat capacity than the portion of the silicon substrate 1a outside the membrane Ma. The difference in temperature is detected by the thermocouple unit T, hence an amount of infrared radiation can be detected.
However, if no signal is provided by the infrared sensor although an infrared radiation may exist, it is difficult to know the cause of providing no signal although the cause may be a disconnection of the thermocouple or some other portion.
The infrared sensor disclosed in JP-A-7-181082 provides a diagnosis circuit that has a infrared ray radiating member under the cold side contact Tc. The infrared ray radiating member is occasionally energized to heat the cold side contact Tc to test the operation of the thermocouple unit T. However, it is almost impossible to carry out such a test until the infrared sensor has been assembled. In addition, the infrared ray radiating member necessitates additional manufacturing steps of the infrared sensor, resulting in production cost increase.